Rear Curtain Sync - this tells your camera to fire the flash at the end of the exposure. When you press the shutter release, your lens will open up, starts collecting light fires up the flash just before it closes.
Front Curtain Sync - this tells your camera to fire the flash at the start of the exposure. When you press the shutter release, the flash will fire immediately and the shutter will remain open, capturing remaining ambient light before closing soon after.
You might not think there’s much difference between these modes but when you’re photographing a moving subject, this will begin to create a dramatic impact. You’ll find many action/sports photographers will use Rear Curtain Sync when shooting with a panning technique.
Try it!